Well, one answer may be improper length, a intake pipe can be tuned. It takes a lot work to tune a intake pipe length, because you have to tune it for the pressure waves, but with a ram effect, it is less critical. When you purchase a new filter housing, talk to a good filter manufacture, one that has race and engine tuning experience. I have found some of the best to be in europe developing filters for formula one and rally. That is wear I aquired my filter setup. Here is simple example on tuning.
The idea here is to continue to use the tuned port advantages in the plenum and intake pipe. Actually, tuned ports are Helmholtz resonators themselves. So you can rear the Tuned Port Basics page to get an idea of how it all works, this page will just take that system further up in the intake track. To make it simple, lets say that there is one throttle bore for a 4 cylinder engine. There will be 2 induction pulses through the throttle bore per revolution. When the air pulses through the throttle bore, is causes a negative pressure wave traveling through the intake pipe. Once this pulse reaches the open end of the pipe (usually at the air cleaner), it will invert to a positive pressure wave. If we can time this wave to arrive back at the plenum to boost pressure when it's needed the most, we may see a power increase. The Helmholtz resonator theory does work well, however, it is limited to how many cylinders can operate off a single plenum. To be effective, no more than 4 cylinders should be used in a single plenum. This set up is very effective on 6 cylinder engine with two plenums, each plenum feeding 3 cylinders. To make matters worse, the cylinders must be even firing, so simply dividing banks of a V6 or V8 will not work unless the banks each fire evenly. For a V8, the best solution is to use a 180 degree crankshaft to even out the firing order of each bank. Then the Helmholtz resonator can be applied as if it were a pair of 4 cylinders. It is possible to see small gains at low rpm with using one plenum for 8 cylinders, but this will usually lead to a reduction in top-end power. There are 3 tunable aspects of the Helmholtz resonator, the plenum volume, intake ram pipe, and intake ram pipe diameter.
Intake Ram Pipe Diameter
This is the easiest to figure out. The velocity in the plenum intake pipe should not be higher than 180 ft/sec at maximum rpm. The formula to figure out the diameter pipe that should be used is for a given velocity is:
D = Square Root of (CID × VE × RPM) ÷ ( V × 1130)
D = Pipe Diameter
CID = Cubic Inch Displacement
VE = Volumetric Efficiency
V = Velocity in ft/sec
If you're dealing with liters, change CID to liters and the constant to 18.5 so the formula will look like this:
D = Square Root of (Liters × VE × RPM) ÷ (V × 18.5)
An example for a 153 cubic inch 4 cylinder with a 85% VE, revving to 6000 rpm would and a desired 180 ft/sec air speed though the intake pipe would look like this:
D = Square root of (153 × 0.85 × 6000) ÷ (180 × 1130) = 1.96
You would need an intake pipe that has a 1.96" inside diameter to have 180 ft/sec air velocity at 6000 rpm for that engine. In other words the engine would need a little over 3 square inches of intake pipe area.
There is not going to be a simple answer to the needed plenum volume for a given application or rpm range. The good thing about plenum volume is that there is a pretty wide range that it can be and still be effective, so general rules work well. The following guide lines are for engine operating in the 5000-6000 rpm rage. V8's with one large plenum feeding all 8 cylinders does not work all that well as far as the Helmholtz resonator goes, but if this is the case, plenum volume should be about 40-50% of total cylinder displacement. For a four cylinder 50-60% works well. For 3 cylinders (6 cylinder engine with two plenums), each plenum needs to be about 65-80% of the 3 cylinders it feeds. If a boost is desired in a higher rpm range, closer 7000-7500 rpm, the plenum will need to be 10-15% smaller. To get a boost in the 2500-3500 rpm range, it will need to need about 30% larger. The plenum size of a Helmholtz resonator may go against the typical plenum size rules, but the rules change when the resonator is being used. The whole Idea of a plenum is to allow the gases to slow down and gain density. The Helmholtz plenum makes a dense charge by use of pressure waves, in the same way tuned port intake runners work. This plenum sizing method does not apply to engines that to not use a tuned intake pipe. Many engine simply have the air cleaner assembly directly on the carb or throttle body having very little intake length. In those cases the Helmholtz resonator system does not work.
Intake Ram Pipe Length
The last thing to adjust is the length of the intake ram pipe. It is possible to make an adjustable pipe that can be made longer or shorter for testing purposes. For a starting point figure a 13" long pipe will help at about 6000 rpm. For each 1000 rpm drop in rpm add 1.7" and subtract 1.7" per 1000 rpm increase. This is just a starting point. The inlet of the pipe should have about a 1/2" radius for smooth flow. Once you get a base line, you must do a power pull and get a baseline. This can be done at the track or on a dyno. They try moving the pipe 1/2" in either direction as see how power improves. The dyno may be a little deceiving, since peak hp my go up but average power may drop. Track testing will be best, since you will be testing in actual racing condition and can tune the pipe for the best times. It is usually best for average power if the intake ram pipe is tuned about 1000 rpm lower than the intake runner length.
Multiple Intake Ram Pipes
Most engines will have more than 1 throttle bore feeding the cylinders. In this case you must figure out the total area of intake pipe needed to figure out what size each pipe should be. In the first example, the 4 cylinder needed a 1.96 diameter intake ram pipe. If that particular engine had a two barrel (or two carbs). You would need two pipes each one having 1/2 the area of a 1.96" pipe. First off, a 1.96" diameter pipe has a total of 3.02 square inches. So we're be looking for pipes that each have 1.51 square inches of area. Using the formula for finding the area of a circle in reverse, you come up with 1.39" diameter. So a pair of 1.39" diameter pipes will act the same as a single 1.96" pipe.
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